As exemplified by A. Abu-Samra et al., Analytical Chemistry, 47, 1475 (1975), and P. Barrett et al., Analytical Chemistry, 7, 1021 (1978), a microwave ovenbased, digestion technique using an acid digesting agent in an open vessel, is known. Barrett et al. report that this procedure decreases digestion time, compared to a hot plate heat source.
Also known, as illustrated by Nadkarni, Analytical Chemistry, 56, 2233 (1984), and S. A. Matthews et al, Bureau of Mines Technical Progress Report 120 (April 1983) is a microwave oven-based, digestion technique using an acid digesting agent in a closed system. In Nadkarni's procedure, a covered Teflon.RTM. or polycarbonate beaker was placed in a partially evacuated, closed, glass desiccator so that acid fumes leaking from the covered beaker were trapped within the glass desiccator. A polycarbonate bottle with a polypropylene screw cap was used as a closed digestion vessel by Matthews et al., who cautioned against explosion caused by pressure build up within a closed vessel and observed leaking of some acid fumes around the bottle caps during pressure dissolution. Matthews et al. report that, compared to open vessel digestion, closed vessel digestion prevents the introduction of contaminating elements and the loss of volatile elements and compounds.
Accordingly, although closed system digestion has been found to have advantages over open vessel digestion, the containing vessel in a closed system could explode under pressure build up, with spewing of acid. The explosion hazard is enhanced, for example, if, as in tissue digestion, gas is generated by the digestion process. As a result, open vessel digestion is commonly utilized, particularly if the digestion process generates gas.
A lidded digestion vessel suitable for use in a microwave system-based digestion that employs a corrosive digesting agent, is available from Savillex Corporation either with or without an outlet port in the lid. This lidded vessel is formed of a deformable, resilient, highly chemically inert, thermoplastic material. This material is a fluorocarbon copolymer in which the carbon-fluorine backbone in the main chain is connected with perfluoroalkoxy side chains through oxygen links. This fluorocarbon copolymer is marketed by Dupont Corp. under the trademark Teflon.RTM. PFA.
Prior art valves are exemplified by U.S. Pat. Nos. 4,474,211 to Lucas, 4,493,444 to Del Bon et al, and 4,400,401 to Beauvais et al. The Del Bon et al patent is directed to a self-closing valve-and-lid assembly. The Lucas patent pertains to a valve that includes a disc-like member through which apertures extend, and an aperture closing device having a pressure responsive flex web with openings offset from the apertures. The Beauvais et al patent relates to a method using microwave energy for sterilizing and canning food products within a nonmetallic enclosure cover having an aperture controlled by a check valve regulated by a weight. Within the enclosure cover, a non-metallic jar with a nonmetallic lid having a vent hole closed by a vent closure, contains the food product.
Also known in the prior art is a manually operated, dispensing valve formed by an opening in a metallic can lid top, a resilient rubber insert located inside the lid top and having an outlet port portion that extends through the lid top, and a plastic pin element. The plastic pin element has a disc-like part with a raised portion that seats against the rubber insert to close the valve, and has a pin-like part that extends through the outlet port portion of the rubber insert for engagement with a nozzle. The valve is opened by finger pressure exerted against the nozzle.
Prior work known to us is a beryllium-copper spring, needle check valve of Lois B. Jassie and H. M. Kingston. This valve attaches to an outlet port of a closed digestion vessel, as a pressure relief valve for explosion prevention. Because the beryllium-copper spring does not absorb microwave radiation, this valve is usable in a microwave system-based digestion procedure. However, a drawback is that beryllium is oxidizable, especially in an acid fume environment, to beryllium oxide, which results in the valve no longer being acceptable for use in a microwave system. Moreover, beryllium oxide is carcinogenic.
Therefore, there is a need for an improved valve that could be used as a pressure relief valve for explosion prevention in a microwave system-based, closed vessel digestion procedure. Such an improved valve would be of even greater usefulness if it were self-closing after excess pressure had been relieved, and thereby could function as more than a one time pop-off valve. Such an improved valve would provide an even greater contribution to the art if it relieved pressure not only in response to a predetermined internal fluid pressure but also in response to temperature change. Such a valve would provide an improved apparatus for microwave system-based, closed vessel digestion.
Moreover, an improved valve of this type would be especially advantageous if it could also be manually opened. This improved valve would be especially remarkable if the components thereof could be of the same non-metallic material. Furthermore, such an improved valve would be of enhanced utility if it had a broad range of applications.